I've done this and there's about three different ones. The really early cars had the left hand driver one and 90 spec cars had one that looks like the mark 3 one which is a self contained assemply. The one you need one of the arms is only connected at one end.
edit I appreciate thats not the best description I'll try and link one when Im not at work. Ebays blocked.
Thats the 3 versions I was thinking about....
The early Left Hand sweep.
Later (Pre 90 (88+)) spec that has smaller wiper boss & 10mm nuts and the
Post 90 Spec (Post 89) that has the larger boss and 13mm nuts...